1. Field of the Invention
This invention pertains to the general field of testing of semiconductor materials. In particular, it pertains to an improvement in the edge electrode of the instrument used for characterizing the properties of electroluminescent semiconductor wafers.
2. Description of the Prior Art
The characterization of semiconductor materials and in particular light-emitting semiconductor structures at the wafer-level (i.e., after forming the p-n junction and the active quantum well layers, but prior to the chip processing steps) is typically carried out with a non-destructive wafer probe and an n-layer contact edge electrode coupled to the wafer. The conductive probe is temporarily placed in contact with the top of the epi-wafer (p-GaN) layer while the n-electrode contacts the n-GaN layer through either the edge of the wafer or through other means that allow access to the n-GaN layer. Such typical layout is illustrated in FIG. 1. When energized, the conductive probe, the semiconductor p-n junction structure on the wafer and the n-electrode form a temporary light-emitting device. By injecting a known current into the junction, light will emit from the device and the spectral properties and their relationship with the electrical properties of the wafer can be measured and characterized.
Although the method of using conductive electrodes in contact with respective layers of the semiconductor wafer for measurements and tests has been known for some time in the field, the issues of making good, consistent electrode-wafer contact with repetitive results have remained problematic challenges that vary from application to application. For example, for light-emitting wafer testing, a well-defined uniform contact area between the probe and the p-layer (GaN) with minimal contact resistance is essential. Therefore, the probe material should be stable under a variety of electrical drive conditions.
One major challenge has been the precise estimation and consistent repetitiveness of the true contact area between the probe and the surface of the wafer, which affect conductivity and all related measurement parameters. A hard metallic probe would be ideal for perfectly flat and smooth surfaces because of the high and uniform conductivity of metals. However, the surface of wafers is typically not perfectly smooth, but it contains a degree of roughness sufficient to create non-uniformities in the way the probe contacts the wafer. Therefore, different probes with softer and elastic tips have been used to cause the probe to deform under pressure and conform to the profile of the wafer's surface. For example, U.S. Pat. No. 7,679,381 (issued to Ma) describes a probe that includes a conductive deformable tip made of elastomer or polymer material and a pressure control that together ensure a good contact with the wafer under test as various measurements are taken across its surface.
Such probes used to improve the uniformity of contact over the test wafer consist of a traditional metallic probe with a conductive silicone tip, such as RTV (Room Temperature Vulcanization) liquid silicone material. Such conductive silicone consists of metallic flakes, typically silver particles, with a nominal diameter in the order of micrometers, dispersed in a silicone carrier that is attached and cured onto the tip of the metal probe. (Depending on the application, other conductive fillers may be used, such as graphite, silver-coated copper, nickel, and so on.) In use, the probe is pressed onto the surface of the wafer, causing its deformable tip to conform to wafer surface irregularities, thereby providing a substantially uniform contact throughout.
While the conductive silicone tip used on a spring-loaded probe as described above produced significant improvements over conventional metallic and other soft-tip probes, certain problems remained unsolved. For instance, when the conductive silicone tip is pressed against the wafer surface, even a very smooth surface, each metal flake incorporated in the silicone tends to make a point contact with the wafer and only silicone material contacts the areas surrounding the points of contact. Thus, while the quality of the contact is uniform in a coarse sense, it is very non-uniform in a microscopic sense and, the conductivity of silicone being negligible in comparison to that of metal, hot spots tend to form in point-contact areas.
As disclosed in U.S. Ser. No. 13/715,486, this problem was addressed and essentially solved by adding a distal metallic layer of micrometer-size particles to an intermediate layer of conformable conductive elastomer material attached to a metallic pin. The resulting probe was able to conduct higher and more uniform currents, with no hot spots and minimal parameter drift during testing.
However, while the above described improvement in the probe produced very consistent and repeatable results from measurements carried out at a given probe location on the surface of the wafer, the results of the measurements varied from different probe locations as a function of the distance between the probe and the n-contact edge electrode. As one skilled in the art will readily understand, the electrical resistance of the wafer's n-GaN layer will depend on the distance between the location of the probe on the surface of the wafer and the point of contact of the edge electrode. As illustrated in FIG. 2, clearly the further the placement of the probe from the edge electrode, the higher the resistance, thereby rendering the measurement results subject to errors related to probe location.
Assuming that M edge electrodes are used with the probe at the center of the wafer, as illustrated in FIG. 3, if the resistance along the n-layer path between the probe and any of the edge electrodes is r, the total resistance R will be approximately equal to r/M (based on the basic formula for parallel-circuitry resistance). Therefore, it is clear that increasing the number of N contact points will significantly reduce the influence of the n-GaN layer resistance on the measurement of a wafer.
Based on this observation, various continuous edge contacts have been developed and tested in an attempt to provide n-GaN layer contact along essentially the entire perimeter of the wafer. However, the uniformity of the contact along the edge remained a problem. The present invention describes a new structure for an edge electrode that provides the uniform conductivity required for reliable and repeatable light-emitting semiconductor wafer testing.